On Wed, 2012-05-16 at 17:33 +0100, Andy Seaborne wrote:
> 1/ Scenario:
>
> A site publishes a page, in RDFa, with triples in it.
>
> The site replaces that page, with one saying, "changes in progress,
> please come back later" with no triples.
>
> Recording this seems like a reason to have empty named graphs.
I agree it's useful, I just don't know how to make it work with
partial-graph semantics. See "subset semantics" in recent message,
http://lists.w3.org/Archives/Public/public-rdf-wg/2012May/0650.html
If
<g1> { }
and
<g1> { :a :b :c }
entails
<g1> { :a :b :c }
then information was the first graph conveying? I think it was
conveying that <g1> had at least no triples, which isn't conveying much
at all.
I think this use case could be addressed with a vocabulary instead of
syntax, as you suggest I think for dealing with quadstores. That is,
instead of allowing in a dataset:
<g> { }
we could put in the default graph something like:
<g> a rdf:EmptyResource.
It's certainly not as elegant, though. (and not compatible with
existing SPARQL systems.)
Oh. I guess with partial-graph semantics (aka subset semantics) one
could also make it complete with a count of the triples. So
<g> { :a :b 1,2,3 }.
<g> rdf:tripleCount 3.
would tell us <g> has those three triples and ONLY those three triples.
With that, the complete-graph-semantics dataset:
<g> { }
could be conveyed by the partial-graph-semantics dataset:
<g> rdf:tripleCount 0.
Still not elegant, but it might be about as simple as possible.
-- Sandro
> 2/ The default graph can be empty.
>
> 3/ A point made has been that N-quads can't represent an empty named graph.
>
> A syntax fix could be:
>
> _ <http://space> .
>
> or other style indicate a space name.
>
>
> To my reading, the spaces document works if "quads" are removed, not
> that I'm suggesting that.
>
> How about defining as a convenience of useful vocabulary rather than
> core definition?
>
> Andy
>
>